WASHINGTON -- Internal Revenue Service documents requested by the House Oversight and Government Reform Committee could take years to produce, the IRS commissioner said.

At a hearing Wednesday, Commissioner John Koskinen sparred with Republicans over the breadth of the investigation into the agency's handling of applications for tax-exempt status of conservative groups.

The Oversight Committee has subpoenaed the IRS for all e-mails to and from key figures in the controversy, including former Exempt Organizations Director Lois Lerner. Lerner has twice refused to testify to the committee, and Chairman Darrell Issa, R-Calif., said e-mails are "the next best substitute."

"You've been more interested in managing the political fallout than cooperating with Congress, or at least this committee," Issa said.

Koskinen said the IRS has provided 690,000 pages of documents to the House Ways and Means and Senate Finance committees, which have primary jurisdiction over the agency and are authorized to view sensitive taxpayer information. House and Senate oversight committees have received 420,000 of redacted documents.

The House Oversight Committee has asked for all e-mails to and from Lerner, her subordinate Holly Paz, General Counsel WIlliam Wilkins and chief of staff Jonathan Davis. It also wants all e-mails to and from any IRS employee and anyone at the White House.

The IRS has been focusing on producing documents with certain key words, like "Tea Party" and "election." If the House Oversight Committee wants all e-mails from those employees, it will get "millions" of e-mails, most of which will be completely irrelevant, Koskinen said.

In one particularly testy exchange with Rep. Jason Chaffetz, R-Utah, Koskinen said the IRS has already spent 10 months and $15 million complying with various investigations. It could take at least 10 more months to comply with a "broader, more sweeping request," he said.

"In a court of law, if you provided that subpoena, a court would not enforce it" because it's too broad, Koskinen said. "We are going to respond to the subpoena. I'm just going to tell you that to comply with this subpoena, we're going to be at it for years, not months."

Democrats said the IRS is cooperating – and that even the Republican chairman of the Ways and Means Committee has said the IRS has taken a "significant step forward" in agreeing to turn over a chunk of Lerner's e-mails. Those e-mails were turned over to the tax committees last week, Koskinen said.

"One has the impression that this is all feeding the base," said Rep. Gerry Connolly, D-Va. "It's designed to get certain groups all riled up before an election."